Individuals with neurofibromatosis type 1 (NF1) carry mutations in the NF1 tumor suppressor gene and develop benign peripheral nerve sheath tumors called neurofibromas which can transform to malignant peripheral nerve sheath tumors (MPNST). Neurofibromas contain normal nerve constituents: axons, Schwann cells, fibroblasts, and mast cells, and aberrant Schwann cells free of axons. Tumorigenesis results from complete loss of function at NF1, as neurofibromas and MPNST are characterized by biallelic mutations in tumor Schwann cells, with other cell types recruited secondarily. We developed mouse model systems for neurofibroma formation in NF1, and also performed large scale gene expression array analyses to identify candidate genes relevant to neurofibroma formation. We also identified a candidate EGFR+ tumor-initiating cell in human and mouse neurofibromas which forms spheres in vitro, can multi-lineage differentiate and form neurofibroma-like lesions in vivo. In this application, we propose to use unique mouse models and the neurofibroma sphere-formation system to test the relevance of this putative tumor-initiating cell to neurofibroma formation. We will also further define markers of the tumor-initiating cells for prospective isolation and to target the cells therapeutically. Finally, we will define molecules necessary for neurofibroma Schwann cell and for putative tumor-initiating cell survival. Together these studies will identify cellular and molecular underpinnings of tumor formation in the nervous system, and identify therapeutic targets for the treatment of NF1.